Figure 1. Possible stages at which selective attention intervenes with processing and the predictions resulting from them. Ai, Early intervention point. Directing attention could differentially modulate the responses of the separate sender populations (“input gain modulation”) as indicated by the orange arrows. The result would be that much stronger signals originating from the attended object would leave the sender populations and thus result in a strong imbalance between stimulus-related signals processed in the receiver population. Aii, Intermediate intervention point. Alternatively, attention could modulate the interactions between receiver and sender populations (“afferent signal gating”) as indicated by the orange cross. The result would be a strong imbalance in favor of processing the attended stimulus' signals in the receiver population, without requiring an imbalance originating from the sender populations. Aiii, Late intervention point. Attention could act at an even later stage by modulating the response of the receiver population, in dependence of its tuning properties, at its output stage (“output gain modulation”) as indicated by the orange arrow. This would change the strength, but not the proportion of the signals of both stimuli processed in the receiver population. B, Sketch of a RF with two stimuli presented that drive the neuron(s) equally well like in our experimental design. This configuration holds for the predictions outlined in Di, Dii, and Diii. C, Sketch of an often used experimental paradigm in which two stimuli are presented in the RF but one is preferred, here stimulus A. This configuration holds for the predictions outlined in Ei, Eii, and Eiii. D, According to input gain modulation (Di), neuronal responses in the receiver population would be the same (black bars; e.g., single neuron firing rates, for a better comparison normalized to the maximum response in the corresponding prediction scheme), no matter which stimulus is attended, because the stimuli drive the neuron(s) similarly. The same holds true for afferent signal gating (Dii) and output gain (Diii). The induced neuronal responses are therefore not a particularly suitable measure for differentiating between the three mechanisms. However, if one would be able to measure the stimulus signals' contribution to the signals generated by the receiver population directly (colored bars; normalized to the maximum contribution in the corresponding prediction scheme), one would expect from the output gain modulation scenario (Diii) that both stimulus signals would be equally well represented, whereas for the afferent signal gating (Dii) mainly the signals from the attended object would be contained. Note, that it is not possible to differentiate strictly between afferent signal gating (Dii, Eii) and input gain modulation (Di, Ei) when only measuring the receiver population. E, Also in the more common experimental setup, in which the two stimuli drive the neuron(s) differently, the neuronal responses (black bars) will not be able to differentiate between the three scenarios. In contrast to D, the receiver population would always show a stronger response when Stimulus A is attended because it prefers Simulus A. Again, when simultaneously measuring both stimulus signals' contribution to the receiver population output (colored bars) one can differentiate between afferent signal gating (Eii) and output gain modulation (Eiii). The proportion between the signal contributions of the preferred and not-preferred stimulus would stay the same for the output gain modulation scenario, as it is merely the gain that changes absolute output strength. In contrast for the afferent signal gating scenario, switching attention would strongly change the proportion between the signal contributions because attention would effectively block the non-attended signals while letting attended stimulus signals through.